In the printing arts, particularly lithography, where multicolor printing is employed, it is necessary, prior to any extended printing run, to ensure the quality of the color separation negatives, or diapositives, from which the printing plates are prepared. While "proof press" practice remains significantly widespread, there has been an increase in the use of "color-proofing" materials and procedures. Contrary to the proof press methods which entail the preparation of actual printing plates for the generation of ink impression prints, the color-proofing materials provide means for the intermediate representation of the color separations which are in great measure sufficiently faithful to the separation photography to serve as indicators of the final printing impressions which might be expected from a printing run.
The more successful color-proofing materials have generally been of two types, comprising either sets of precolored or pigmented photosensitive sheets which are exposed to the set of color separation originals and processed to provide a series of colored proof images which superimposed yield a multicolor representation of the final work, or photosensitive sheet materials which are substantially colorless yet respond to photographic exposure to yield latent images which may be colored respectively with the primary hues in the form of pigments, inks, or dyes.
The former of these two types of color-proofing materials, i.e., the precolored sheets, are generally deemed less advantageous in view of the fact that the depth or intensity of color cannot readily be varied. The requirement for a large inventory of variously colored sheet materials further detracts from their utility. For these reasons it has been recognized that the proofing materials which can be employed in a post-coloring process are better adapted to the lithographer's needs in providing preliminary reproductions of the color separation originals in those tones and variations of color intensity which will be required in the final printing.
Among the post-colored proofing materials are those described in U.S. Pat. No. 3,060,024 where a photopolymer exposed to render light struck areas less tacky under applied heat is developed by the application of a finely-divided process pigment, the pigmented image being transferred with heat to a receptor sheet in register with other prints of the set; and in U.S. Pat. No. 3,634,087 where a diazo resin layer, exposed to a color separation original, is developed with an alkaline fluid to remove unexposed areas and yield an image which is colored with an ink/lacquer emulsion, the imaged proof sheet then being superimposed in registry upon other proof sheets of the set.
Particularly useful methods for preparing surprint proofs of broad utility and exceptional fidelity are described in U.S. Pat. No. 4,174,216 and the above-identified related application Ser. No. 795,339, and generally comprise the imagewise removal of portions of an overlying and non-tacky masking layer from a supported, tacky adhesive layer with subsequent toning or coloring of the revealed adhesive image by the application of colored powder. A plurality of such imaged layers in different colors may be, in register, formed on or laminated to film, foil, or paper stock to provide the surprint proof.
In order to protect the surface of the surprint proof, it is commonly the practice to laminate to such surface a thin transparent film of polyester or the like, or a layer of photopolymer which may be polymerized by overall exposure to actinic radiation to form a tough, protective membrane. These prior practices, however, generally result in the formation of a smooth, glossy surface on the surprint which in some circumstances detracts from the desired appearance of the proof. In many instances, for example, it is preferred that the surface of a multilayer proof print exhibit a matte, or non-glossy, texture in order to closely resemble actual printing on stock which lacks a coated or natural sheen.
Various means have been attempted to reduce the glossy appearance of the noted type of surprint proof, yet none has proven sufficiently simple and effective. The application to the surface of the print of powdered pigments, pigmented sprays or lacquers, or aqueous dispersions of pigments, all as mentioned, for example, in U.S. Pat. No. 4,286,046, poses problems of expense, cleanliness, and health while failing to provide a matte of acceptable uniformity, texture, and appearance.